This invention relates to the desulfurization of coal and more in particular to a process of reducing the pyritic sulfur content of coal.
The energy crisis has made inevitable an increasing use of high sulfur coal as fuel. At the same time, unexpected difficulty in scrubbing SO.sub.2 from stack gases has spiraled projected costs of controlling air pollution. Furthermore, it has been estimated that the amount of sulfur oxide produced from the combustion of coal will increase from 26 .times. 10.sup.6 metric tons annually in 1966 to 55 .times. 10.sup.6 metric tons annually by 1990. As a result, there is increased interest in techniques for removing sulfur from coal prior to combustion.
Coal is generally classified in four groups: 1 anthracite, 2 bituminous, 3 sub-bituminous, and 4 lignite. The sulfur content of these coals is broadly divided into three classes; pyritic (FeS.sub.2), organic sulfur molecularly bound to the coal matrix, and sulfate. The last named occurs only in weathered coal and can be essentially neglected when dealing with freshly mined coal. In a typical grade of coal, about half the sulfur is present as pyrite in simple mixture with other constituents; the rest is locked into organic molecules.
Many methods for removing the sulfur in coal prior to combustion have been devised. One method, described in The Journal of Industrial and Engineering Chemistry, 12, 1069 (1920), involves the treatment of coal at elevated temperatures and atmospheric pressure with hydrogen gas. During hydrogenation the pyritic sulfur is reported to form a species resistive to subsequent acid treatment. The process of U.S. Pat. No. 2,726,148 discloses a process of treating coal at both an elevated temperature and at an elevated pressure with hydrogen.
One disadvantage of these simple hydrogenation processes is that in most cases only about 50% by weight of the recoverable sulfur initially combined as pyritic sulfur in the coal can ultimately be removed. It is desirable that a process be developed to increase the amount of pyritic sulfur that can be removed from coal, thereby reducing the total sulfur content of the coal.